(a) Field of the Invention
This invention relates to an infrared intrusion detector useful in monitoring a corridor-like room comprising an infrared sensor for detecting a change of infrared radiation impinged on the infrared sensor by a passing intruder, a plurality of optical means mounted in front of said infrared sensor for receiving infrared radiation from the body of said intruder and focusing said radiation on said infrared sensor, and an evaluation means coupled to said infrared sensor for actuating a signal when said infrared sensor detects said radiation change.
(b) Discussion of the Prior Art
Infrared intrusion detectors are generally known; they detect the intrusion of a person or any object emitting infrared radiation in a supervised area.
For the supervision of corridor-like rooms, specially adapted infrared intrusion detectors are used having a relatively broad field of view in one plane and a relatively narrow field of view in a transverse plane. The broad field of view is usually in the vertical plane, with the narrow field of view being provided in the horizontal plane such that a curtain-like protection zone is provided. The protective curtain is arranged within a facility to be monitored such that an intruder must traverse this curtain to gain entrance into the facility and thereby trigger an intruder alarm. GB-A-No. 2,080,945 describes an infrared intrusion detector in which such a curtain is produced by a cylindrical mirror which is placed in front of the focusing mirror in order to obtain a wide vertical angle of view.
This infrared intrusion detector has a disadvantage in that it has a different sensitivity for objects in areas having different ranges from the detector.
In DE-Al-No. 31,14,112, a detector system based on infrared radiation is described, which avoids said disadvantage and achieves an approximately equal level of sensitivity to infrared radiation for all areas having different ranges from the detector. This is achieved by arranging three vertically displaced concave mirrors with an infrared sensor in their common focal point in such a way that each mirror provides coverage for a different angular region of space. For a given object (e.g. a person), each of the mirrors focuses an image of said object upon the sensor having substantially the same image size independent of the distance of said object from the detector. An object of a given size emitting infrared radiation is therefore detected approximately with the same probability of detection, and the sensitivity of the detector is approximately equal for all areas of coverage independent if their distance from the detector.
A disadvantage of this known infrared intrusion detector arrangement consists in the fact that the area to be supervised is not covered completely. Because of the gaps between the coverage areas dictated by the optical constraints, especially in front of the detector, such infrared intrusion detectors are not sufficiently safe against sabotage or against crawling intruders.
In EP-Al-No. 0'262'241 (corresponding to U.S. Pat. No. 4,740,701), it was suggested to provide an infrared detector having a field of detection in the form of sharply defined strips or elongate zones of substantially uniform sensitivity to infrared radiation without a gap by bending a thin cylindrical Fresnel lens in the longitudinal direction in such a way that the radius of curvature corresponds to its focal length. The infrared sensor is arranged approximately in the focal point of thus created cylindrical Fresnel lens. An advantage of this arrangement is that a protective curtain without a gap is obtained, but the disadvantage is that the sensitivity of the detector decreases with increasing distance from the detector. (The sensitivity of the detector is approximately inversely proportional to the distance from the infrared intrusion detector; see FIG. 7.)